1. Field of the Invention
The present invention relates to a photoacoustic cell and to a photoacoustic measuring device constituted by using the same. More specifically, the invention relates to a photoacoustic cell and a photoacoustic measuring device that makes it possible to make in-vivo and in-situ measurement. Particularly, the invention relates to a photoacoustic cell and a photoacoustic measuring device that are expected to be used for a highly sensitive and highly precise measuring device required in the fields of measuring the amount of moisture in human skin and measuring the percutaneous absorption of medicines.
Furthermore, the invention relates to a photoacoustic cell and a photoacoustic measuring device which finds a wide range of applications using either the open type cell or the closed type cell and which, when a sample holder is optionally attached, can serve as a closed type photoacoustic measuring device, and which is small in size and is easy to use compared with the conventional closed type photoacoustic measuring devices and which is further expected to provide improved sensitivity and precision.
2. Description of the Related Art
A so-called non-destructive analytical method which qualitatively or quantitatively analyzes the materials to be measured without destroying them is becoming more important than ever before in the field of analysis of biological samples.
The conventional non-destructive analysis generally consists of irradiating a sample to be measured with light and measuring the reflected light or the transmitted light.
In the case of a biological sample having a rugged surface and strong scattering property, however, it is difficult with the conventional method to correctly measure the energy that is absorbed or reflected. This is because when the light absorption characteristics are to be measured, the light being irradiated decreases due not only to the absorption of light but also to the scattering of light and when the reflection characteristics are to be measured, correct measurement is hindered by the light scattered by the rugged surface.
In recent years, therefore, a photoacoustic method has been developed according to which the energy absorbed by a material being inspected is not measured as light but is measured as a pressure wave (sound wave) produced due to the generated heat. This has been applied to measuring the light absorption characteristics of materials having strong scattering properties such as biological samples since this method is little affected by the scattered light.
Here, the factor that dominates the amount of the generated heat is called the thermal diffusion length (.mu.) and is expressed by the following formula. EQU .mu.=(2k/.rho.c.omega.).sup.1/2
k: thermal conductivity of the material, PA1 .rho.: density of the material, PA1 c: specific heat of the material, PA1 .omega.: angular modulation frequency of the irradiated light.
Therefore, when k, .rho., and c remain constant for a given material, the thermal diffusion length changes with a change in the frequency of the irradiated light, and the amount of the generated heat changes if there exists a material having thermally different properties in the region of the thermal diffusion length.
By changing the modulation frequency of the irradiated light, therefore, it becomes possible to effect the analysis in the direction of depth in the level of thermal diffusion length. Furthermore, since the intensity of signal varies in proportion to the intensity of the source of light, even those materials that absorb small amounts of light can be measured.
In the ordinary photoacoustic method, it is necessary to process the sample by cutting the sample so that it is placed and measured in a small closed type cell. In order to carry out in-vivo and in-situ analyses, the photoacoustic analytical method and apparatus using an open type cell have been reported according to which one surface of the cell is opened and an air-tight system for the material to be measured is constituted on the open surface. At present, therefore, the closed type cell and the open type cell are used depending upon the samples and the applications. Compared with the closed type photoacoustic cell, however, the open type photoacoustic cell is affected by the environmental noise and gives large noise components and low sensitivity. In order to decrease the effect of noise components there has been proposed a method (P. Poulet, J. Chambron, J. Photoacoustics, 1, 329-346 (1983)) which uses two photoacoustic cells, i.e., a cell of the measuring side which is irradiated with light and a cell of the reference side which is not irradiated with light but measures the noise components only, in order to detect the difference between the two signals using a differential microphone. However, the intensity of the photoacoustic signal detected by the cell of the measuring side has not been measured at a resonance frequency and is weak since the light signal is not strong. Moreover, light beams irradiating the cell of the measuring side and the cell of the reference side have different phases, the noise is not completely erased by the differential microphone, and the sensitivity is not improved.
In order to improve the intensity of the photoacoustic signal by bringing the modulation frequency of the irradiated light into agreement with the resonance frequency of the cell, furthermore, there has been reported a method (Kolmel, K.; Sennhenn, B.; Giese, K. J. Soc. Cosmet. Chem. 1986, 37, 375-385) which uses a resonance type cell of the measuring side and a cell of the reference side to make measurement after the noise has been removed by using a differential microphone. However, the cell of the measuring side and the cell of the reference side have different resonance frequencies and different noise components. Therefore, the difference is not perfect and good sensitivity is not obtained; i.e., it is desired to develop a more sensitive device.
There has further been disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-27215 a photoacoustic measuring device having an open type cell. According to this conventional photoacoustic measuring device constituted by a cell of the measuring side and a cell of the reference side, either one of the microphones is mounted by using a threaded block.
The capacity of the cell is adjusted by the threaded block to control the resonance (Helmholtz's resonance) frequency of the cell.
Therefore, the above conventional device makes it possible to intensify the optimum signals at the resonance frequency.
Furthermore, use has heretofore been made of a xenon lamp or a deuterium lamp as a source of light for the photoacoustic measuring device, and the monochromatic light is obtained through a spectroscope and is guided to the photoacoustic cell through the light guide. Therefore, the light falling on the sample from the light guide has optical intensities at each of the wavelengths of the order of nW to .mu.W, and the obtained photoacoustic signals are as weak as several tens of nV. In order to obtain these signals, a complex signal analyzing technology (signal analyzer, analytical software, or the like) is necessary.
However, the conventional photoacoustic (hereinafter referred to as PAS) measuring technology using the open type cell develops the following problems.
(1) Since the intensity of the source of light (particularly ultraviolet light) is weak, the obtained PAS signals are so small that the measurement is not obtained with a high sensitivity. That is, with the xenon lamp or the deuterium lamp used so far as the source of light, the intensity of light per a wavelength is small. A laser beam must be used.
(2) The conventional open type PAS cell has not been optimized in its constitution from the standpoint of amplifying the PAS signals or removing noise and further has a large dead volume (about 1 ml), making it difficult to obtain measurement with a high sensitivity. That is, the photoacoustic signal consists of a sound wave which changes with a change in the pressure and it is considered that the sensitivity increases with a decrease in the volume of the cell. With the conventional cell structure, however, it is difficult to decrease the dead volume. It is therefore urged to study the cell structure which minimizes the dead volume. This holds true even with the conventional closed type cell.
(3) In carrying out the analysis in the direction of depth, it is not allowed to optimize the sensitivity depending upon the conditions. That is, the data in the direction of depth is obtained by changing the frequency of photoacoustic measurement. In the case of the conventional photoacoustic cell, however, the sensitivity is simply improved at only one measuring frequency owing to the effect of resonance. To carry out the analysis in the direction of depth with a high sensitivity, therefore, it becomes necessary to use a photoacoustic cell having a structure that is capable of freely developing resonance at any frequency of measurement.
In recent years, furthermore, it has been demanded to provide a measuring device with high sensitivity and high accuracy in a variety of fields such as measuring the amount of moisture in human skin and measuring the percutaneous absorption of medicines. At present, however, a suitable device has not been developed.
As a method of evaluating the percutaneous absorption of medicines, for example, there has been proposed the in-vivo measuring method using an RI (radioisotope) or the like. This method, however, requires a complex measuring system and is cumbersome to use. Usually, therefore, the in-vitro measuring method has been put into practical use by employing a relatively simply constructed diffusion cell.
Though a variety of diffusion cell methods have been developed to bring the conditions closer to those of actual living tissue, it has still been strongly desired to develop simple in-vivo and in-vitro evaluation methods with a high sensitivity and high accuracy.
Furthermore, a variety of lasers have been developed as sources of light for use in infrared, visible, and ultraviolet regions having very greater output intensities than those of the generally used incoherent xenon lamps and deuterium lamps. Use of these lasers as a source of light for the photoacoustic measuring method makes it possible to achieve high sensitivity and high accuracy. Though there has not yet been provided an ultraviolet laser that continuously emits light in the ultraviolet region, it is possible to continuously emit ultraviolet light if a laser that continuously emit light such as argon laser or krypton laser is combined with a nonlinear optical crystal. This is expected to be widely adapted to medicines.
It is further considered to be effective in studying the percutaneous absorption of medicines based on the analysis in the direction of depth, which is a feature of the photoacoustic method.
Further, the closed type photoacoustic cell that is now generally used is a system in which a sample is introduced into a sample holder made of quartz and is hermetically closed in the cell, having a defect of a relatively great dead volume. If the dead volume can be decreased, the sensitivity is improved correspondingly. It is therefore necessary to study the constitution of a closed type cell having a small dead volume. It is further desired that the closed type cell can be easily used.
The conventional photoacoustic measuring device employs the closed type cell or the open type cell depending upon the object of measurement and, hence, two devices and systems therefor must be assembled each time. The range of application will spread greatly if there is available a photoacoustic measuring device equipped with a photoacoustic cell which can be easily switched to that of the closed type and that of the open type. It is therefore urgently desired to improve the photoacoustic measuring device.